1. FIELD OF THE INVENTION
This invention relates to the field of digital transactions, particularly to transactions in which copies of digital media files are acquired. The media files are typically music files (but can be any other type of media file) and the delivery mechanism may be, but is not limited to, a digital radio transmission such as DAB (Eureka-147) or the Internet.
2. DESCRIPTION OF THE PRIOR ART
Conventional analogue radio systems (such as FM or AM) can cause significant channel noise to be introduced into the transmitted audio signal payload; the payload is in any event generally rendered into the analogue domain before being allowed to modulate the carrier. As a result, analogue systems are inappropriate vehicles for selling music directly, since any recording will be significantly compromised with respect to the (in these days, usually digital) original which can be purchased through conventional music stores (e.g., as a CD) or through the Internet (e.g., as an MP3 file).
However, with the introduction of digital radio systems (e.g., the Eureka-147 standard adopted throughout the UK, Europe and elsewhere) the possibility of the direct sale of digital music ‘over the air’ appears, at first, to become a significant possibility. This is because:                Digital sources are used. The source music is encoded using a digital compression system (e.g., Musicam (a slight variant of MPEG1-layer-2 and MPEG2-layer-2 audio) for Eureka-147 DAB). Therefore, there is little, if any, perceptible difference between the material put to air and that which can be purchased by the user though the normal commercial channels, as described above.        The received signal will be, for most users, an exact copy of that which was transmitted. Modern digital radio systems all contain some form of forward error correction (FEC). This involves the addition of structured redundancy to the transmitted signal so that the receiver can accurately infer the intended payload message, even in the face of significant corruption by the channel. Coupled with checksum tests on the audio, and channel error temporal and frequency decoherence mechanisms, this means that digital radios tend to have so-called ‘cliff-effect’ reception—either the received audio is exactly that which, bit-for-bit, left the studio, or no signal at all is received. This must be contrasted with analogue radio, in which most channel corruptions become directly perceptible within the delivered audio signal.        Potential lack of source compression. Analogue radio systems often artificially compress the source material spectrum in order to make their signal appear ‘louder’ (useful e.g., for in car listening where the ambient noise level is high and quiet passages can be lost). Digital radio systems can (although they need not) avoid this problem, through the use of mechanisms such as the Eureka-147 dynamic range control, in which compression is performed at the receiver end.        
These three points, taken together, mean that the digital radio user receives, essentially, a digital ‘clone’ of the original message. This is (minus packaging) what the user would be able to purchase through the normal channels discussed above, hence the possibility of commercial sale of the material ‘downloaded’ over the air. The commercial sale of material ‘downloaded’ over the Internet already occurs of course.
However, using radio as the vehicle for the commercial sale of music turns out not to be practicable using existing known mechanisms, in the general case, for the following reasons:                Radio stations tend not to transmit all of a particular music item—for example they will often ‘cut’ the start and ends of popular music tracks to make them fit a particular play slot. (This is also a problem with ‘classic request’ channels which may play only one movement from a classical piece, or worse, a selection from a single movement).        The radio station will often impose corruptions onto the transmitted item—for example, ‘fades’ at the start and end of a track, the disk jockey talking over certain parts of a track, special effects, adverts or jingles overlaid on the track, etc.        Even if the above two factors were not a concern, then piracy would prove problematical for the music vendor. This is because, in order for the radio to play the track, the music will have to be transmitted, to all intents and purposes, ‘in the clear’. However, for security, the track would need to be encrypted, only to be unlocked by an authorised purchasing key.        